This application claims the priority of German application 198 13 012.0, filed Mar. 25, 1998, the disclosure of which is expressly incorporated by reference herein.
The invention takes its departure from a method for manufacturing a hollow body from a tubular blank by internal high-pressure shaping.
Previously, round or rectangular tubular sections had to be pre-shaped as blanks for the internal high-pressure shaping process as a function of the workpiece, with pre-shaping taking place in a separate pre-shaping tool or in the internal high-pressure shaping tool. However, the only solutions known in this connection are those in which the smallest circumference of the finished part corresponds to the circumference of the blank to be inserted, in other words the blank is dimensioned according to the smallest circumference of the finished part.
An exception is the pinching off of areas of the tube when the internal high-pressure shaping tool is closed. A kink with twice the wall thickness of the raw material then forms on the exterior of the tube. The space requirement for the kink has proven to be problematic. Separation of the kink would entail an additional workstep; in addition, the torsional rigidity of the finished part would be adversely affected.
Since the circumference of the tube to be inserted as a rule corresponds to the minimum circumference of the finished part, the possible part cross sections can be varied only within narrow limits. For example, if the part to be made has a circumference at the smallest point of 100 mm and 150 mm at the largest point in the finished state, a blank with a circumference of 100 mm is usually used. This means however that areas with a large circumference in the circumferential direction must be expanded by approximately 50% for example. This is not possible with ordinary metal materials. In the past, when it was necessary to produce internal high-pressure shaped parts with greater differences in diameter, the only option was to perform the shaping in several shaping steps and to regenerate the shapability of the material by intermediate annealing, which is very costly in terms of investment outlay, space requirements, and operating costs. An alternative is to use blanks that have been assembled from a number of parts (so-called tailored tubes) which are welded together from tubular sections with transition cones of different sizes. This involves expensive and time-consuming manufacture of blanks.
A goal of the invention is to improve the basic method of the above described type in such fashion that finished parts can be produced with larger differences in diameter or circumference (more than 15% for example) between the largest and smallest diameters using metal materials in conventional use today simply by internal high-pressure shaping.
This goal, on the basis of the basic method is achieved according to the invention by a method of the above noted type, wherein, for pre-shaping of the blank, a dent whose depth varies over its length is formed in the blank, said dent being configured so that a U-shaped fold that projects into the interior of the hollow body is formed with the walls of said fold abutting one another directly and merging at the periphery with a circumferential wall of hollow body to thereby facilitate the internal high-pressure shaping of the pre-shaped blank to form a hollow body with a circumference that varies sharply over its length.
During the pre-shaping of the blank, a dent is formed whose depth varies over its length, from which a U-shaped fold is formed during the actual internal high-pressure shaping process that projects into the interior of the hollow body and whose walls abut one another directly and blend at the periphery with the circumferential wall of the hollow body. Thus, a blank can be produced that is larger in circumference than the smallest circumference or smallest diameter of the finished part. On the basis of the fold, which is directed inward and hence does not pose any difficulties, circumferential material can be "used up" and thus the diameter or circumference of the blank can intentionally be reduced locally. As a result, larger blanks can be used whose circumferential length or diameter is between the maximum and minimum diameters of the finished part. As a result, the circumferential expansion of the workpiece in areas with a large finished-part circumference can be kept within tolerable limits. The range of parts that can be manufactured with conventional simple blanks is considerably expanded, thanks to the invention.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.